1. Technical Field of the Invention
This invention relates to call processing in a telecommunications system and, more particularly, to the processing of an enhanced Call Forwarding Don't Answer feature.
2. Description of Related Art
Developments and improvements in communications switching systems have allowed telecommunication system operators to offer a number of new and useful services to system subscribers. One example of this type of service is the call forwarding feature currently provided by telephone companies. Call forwarding allows subscribers to the service to automatically redirect calls made to the subscriber's number (base address) to another number (remote address) in the system.
Present telephone systems provide the call forwarding feature in various forms. Examples include Call Forwarding Variable (CFV), in which the subscriber is allowed to directly activate and deactivate forwarding and to specify the remote address to which calls are to be forwarded during each activation. While CFV is active calls to the subscriber's line are directed to the remote address, and, if the remote address is idle, a reminder ring is provided to the base address. A second example of call forwarding features is Call Forwarding Busy Line (CFBL). In CFBL, calls to the feature base address are forwarded to a remote address only when the base address is busy. A third example of call forwarding is Call Forwarding Don't Answer (CFD). In CFD, calls terminating at the base address ring the base address for a specified interval. A CFD timer is used to time the interval. If the call is not answered, it is routed to a remote address.
Because the base address rings in CFD, present implementations of CFD suffer from a shortcoming which occurs when calls cannot be successfully forwarded. When a call is not successfully forwarded after the called base address hasn't answered, a caller (originating address) will hear a ring tone and then a busy or congestion tone. Hearing the busy or congestion tone after having heard a ring tone is confusing to the caller and is not desirable in a telephone system.
A typical solution involves checking the line state of the forwarding line before the call is forwarded. If the line state is other than idle, the call is not forwarded. This solution is disadvantageous in that the remote address may be busy but may have a feature, like Call Forwarding Variable (CFV), Call Waiting (CW) or Series Completion Busy (SCB), that could enable the call to be completed, but will not be invoked because the call is not forwarded upon finding the line other than idle. Also, a second problem occurs when, even if the line state is idle, a call can not be forwarded successfully due to congestion. In this situation the caller at the originating address will hear the confusing sequence of a ring tone followed by a congestion tone.
The telephone industry standard for Call Forwarding Subfeatures, Bellcore document TR-TSY-000586, Issue 1, July, 1989, is hereby incorporated by reference herein. The standard requires that the problem of unsuccessful forwarding to a second feature or busy remote address be accounted for in systems conforming to that standard. The BellCore standard sets forth the general requirement that if the remote address is busy, but has a forwarding feature that can be invoked, then the forwarding from the remote address should be performed. The standard also requires that if the remote address is busy, and has no forwarding features, then the call should be considered a non-forwarded call, and non-forwarding call treatment is applied to the call. The non-forwarding call treatment in the standard is to continue ringing at the base address and continue sending a ring tone to the originating address. If the standard requirement is met, an originating subscriber will not hear a ring tone followed by a busy tone if the remote address is busy. Also, a second forwarding will be performed if the CFD is to a remote address which is assigned a forwarding feature.
The Bellcore standard does not address the problems which can occur in CFD when sequential forwardings are involved. For example, an originating address may call a base address, which in turn has CFD implemented to forward to a first remote address which has CFV implemented to forward to a second remote address. In this situation if the base address does not answer, and the second remote address is busy, the caller at the originating address will hear a ring tone when the base address rings and then hear a busy tone after the call is forwarded from the first remote address to the second remote address. In addition, a second problem occurs in this solution when a call can not be forwarded from the base address successfully due to congestion, even though the remote address is idle, or is busy and has a feature that can be invoked. In this situation, even though the remote address is idle or is busy and has a feature that can be invoked, the caller at the originating address will hear the confusing sequence of a ring tone followed by a congestion tone.